Crystal structures and luminescence properties of novel compounds K4M2[Al2Si8O24] (M = Ce, Gd)

Two novel potassium rare earth silicates, obtained by hydrothermal synthesis, have been investigated by X-ray diffraction and described by the general formula K 4 M 2 [Al 2 Si 8 O 24 ] ( M = Ce, Gd). The parameters of the monoclinic K 4 Ce 2 [Al 2 Si 8 O 24 ] and K 4 Gd 2 [Al 2 Si 8 O 24 ] cells are, respectively, as follows: a = 26.867(1), 26.6520(2) Å; b = 7.4150(2), 7.2854(1) Å; c = 14.910(1), 14.8182(1) Å; β = 123.52(1)°, 123.46(1)°; and sp. gr. P 2 1 / n . The structures are solved by the charge flipping method and refined in the anisotropic approximation of thermal vibrations for atoms to R = 5.2 and 2.5%, respectively. The compounds under study are crystallized into a new structural type, which is based on two-level [Al 2 Si 8 O 24 ] layers, combined into a three-dimensional framework by columns of edge-sharing seven-vertex REE polyhedra. Potassium atoms are located in the framework channels. The spectral luminescence characteristics are determined. The luminescence bands are typical of Gd 3+ and Ce 3+ ions. Upon excitation by light with λ exc = 246 nm, a band due to the 2 D → 8 F 5/2 transition with λ max = 430 nm is observed in the spectrum of K 4 Ce 2 [Al 2 Si 8 O 24 ] and a band related to the 6 P 7/2 → 8 S 7/2 transition with λ max = 311 nm is observed in the spectrum of K 4 Gd 2 [Al 2 Si 8 O 24 ].